1. Department of Mathematics and Statistics, College of Science, Taif University, P. O. Box 11099, Taif 21944, Saudi Arabia
2. Department of Mathematics, Faculty of Science, Zagazig University, P. O. Box 44159, Zagazig, Egypt
3. Arab Academy for Science Technology and Maritime Transport, P.O. Box 1029, Alexandria, Egypt
4. National Committee for Mathematics, Academy of Scientific, Research and Technology, Egypt